There has been a recent trend in the use of electrochemical capacitors for enhanced storage of electrical energy. These capacitors derive their enhanced characteristics from two primary mechanisms: double layer capacitance and pseudocapacitance. Double layer-type capacitors use an electrical double layer (explained below) to achieve a very small charge separation (d), which increases electric field (E) for a given voltage, increases capacitance (C) and consequently increases the energy stored (U) for the given voltage versus a conventional planar surface capacitor, as apparent in Eqs. 1 through 3 below.
                    E        =                  V          d                                    Eq        .                                  ⁢        1                            where E=electric field, V=potential difference or voltage, and d=separation of charged plates.        
                    C        =                              k            ⁢                                                  ⁢                          ɛ              0                        ⁢            A                    d                                    Eq        .                                  ⁢        2                            where k=relative permittivity or dielectric, C=capacitance, ∈0=permittivity of free space, and A=cross-sectional surface area.        
                    U        =                              1            2                    ⁢                      CV            2                                              Eq        .                                  ⁢        3                            where U=energy stored, C=capacitance and V=voltage.        
Practically, the smaller thickness (d) allows for much more surface area of the plates to be packaged (usually rolled or stacked) in a given volume. As evident from Eq. 2, this area increase also significantly increases capacitance. Devices of the above described nature are commonly referred to as electric double layer capacitors (EDLCs).
In pseudocapacitors, which are a hybrid between double-layer capacitors and batteries, both the bulk and the surface of the material play key roles. They thus can store much more energy than conventional planar surface capacitors, but face many of the same reliability and scientific challenges as advanced batteries, including high cost due to expensive raw materials and complex processing. Pseudocapacitance imitates battery technology by storing energy in chemical reactions (oxidation and reduction) which take place at or very near the surface of the relevant electrodes. The surface nature of the reactions is the distinguishing characteristic from chemical battery technology.
Either or both of these effects (i.e., double layer and pseudocapacitance) may be used in so called “supercapacitors.” Advantageously, the invention herein makes use of and extends double layer theories in a novel manner, without any formal “chemical reactions” present.
Also previously explored is the notion of enhancing a double layer capacitor by the application of an electrically conducting polymer e.g. Hu, U.S. Pat. No. 8,164,881. While the invention described herein certainly makes use of a polymer coating, the polymer is sometimes electrically resistive and sometimes insulating but is not electrically conducting by design. This significantly differs in structure, nature and consequently in function from previous applications.
Current EDLCs can handle only low voltages before breakdown. In order to attain the higher voltages necessary for many practical applications (such as electric vehicles), low voltage EDLCs are connected in series much in the same way batteries are series-connected for high voltage use. An energy storage device constructed according to principles of the invention can handle higher voltages and be connected in series.
The invention is directed towards overcoming one or more of the fundamental problems with existing designs and solving one or more of the needs as set forth above.